1. Field of the Invention
The present invention generally relates to the field of thermal barrier coating materials. More particularly, the invention relates to the field of corrosion resistant thermal barrier coating materials.
2. Background Art
Metallic components that operate at high temperatures, such as industrial gas turbines or aero-engine parts, often utilize a thermal barrier coating to keep metal temperatures low and prolong the life of the component. Insulating thermal barrier coatings allow the metallic components to operate at higher temperatures, which improves efficiency. At high temperatures, stabilized zirconia has a tetragonal or cubic crystal structure with a very low thermal conductivity, which makes it ideal for high temperature thermal barrier coating applications. However, when zirconia cools from these high temperatures, it transforms from a cubic to a tetragonal to a monoclinic crystal structure, causing a volume expansion that induces high stress and affects the integrity of the coating. Conventional thermal barrier coating solutions have focused on stabilizing the cubic phase of zirconia.
Moreover, impurities in fuels, such as sodium, sulfur, phosphorus, and especially vanadium, can corrode a thermal barrier coating and cause spalling of the coating off the metallic component. This is especially prevalent in industrial gas turbine applications, which do not use fuel as “clean” as that used for aero-engines. Corrosion of the thermal barrier coating exposes both the thermal barrier coating and the metallic component underneath to not only to the corrosive fuel impurities but also to high temperatures, drastically reducing the lifetime and efficiency of the metallic components and the thermal barrier coating.
Prior efforts to stabilize the cubic phase of zirconia while making the coating chemically resistant to the corrosives found in many fuels has proven difficult. One conventional method utilizes scandia stabilized zirconia as a thermal barrier coating, which results in an effective thermal barrier coating with high resistance to corrosives. However, scandia is extremely expensive, making this method impractical for widespread use. Another conventional method attempts to utilize indium stabilized zirconia as a thermal barrier coating. However, indium is so volatile that the indium in an indium stabilized zirconia material boils off during plasma spraying or electron beam physical deposition and does not remain in the thermal barrier coating. Thus there is a need in the art for a cost effective corrosive resistant thermal barrier coating material for use in high temperature applications without the shortcomings of conventional corrosive resistant thermal barrier coating materials.